This invention relates to the detection of audio signals from input pulses that have been encoded with continuously-variable-slope delta (CVSD) modulation. In particular, it is an improved CVSD modulator that is especially adapted for use in integrated circuits using complementary metal-oxide semiconductors.
CVSD modulation is a well-known scheme for converting analog signals to digital form. This is a non-linear system in which the direction of the slope error is determined and subjected to a rule. If the slope error is in the same direction for a predetermined number of pulses (the rule), then the system is caused to change the slope of the delta-modulated signal. This system holds the changed slope at the new value until the rule indicates that it is time to change the slope. By this means it is possible to have a continuously adjusted slope that is the minimum amount necessary to follow the audio signal. This maximizes the closeness with which the delta-modulated signal follows the audio signal while at the same time minimizing the amount of pulse noise in that signal. The foregoing statements are well known and are illustrated in a number of references. Two typical such references are a textbook, "Delta Modulation Systems," by R. Steele, and U.S. Pat. No. 4,151,517 entitled "closed Loop Companding Ratio Control for Continuously Variable Slope Delta Modulation," assigned to the assignee of the present invention.
Three different types of noise are characteristic of a CVSD system. They are the idle signal, quantization noise, and beats. The idle signal, is a sawtooth or other periodic wave typically at 6 kHz which is half the bit rate. It is produced by alternating information bits during intervals when there is no signal. Quantization noise is an error signal produced by the difference between the reconstructed signal and the analog signal from which it was derived. Beats are products of input frequencies and the idle frequency. It is common practice to filter out this component and any other components outside the audio bandwidth that are in a CVSD receiver by applying the reconstructed signal to a filter. This is normally done in an analog filter which is placed at a point after the CVSD demodulator where the signal has already been reconstructed into analog form. Removal of the unwanted components from the CVSD process improves intelligibility of the reconstructed signal.
When CVSD modulation is used in a communications receiver, the filter will normally be a low-pass filter with a cutoff at a frequency of the order of 3 kHz. This is a compromise that maximizes voice signals while minimizing distortion. Communications receivers are restricted in bandwidth, which requires a relatively low bit rate for digital signals. This causes the noise components to be in a frequency band that can be heard and hence that will affect intelligibility. The relatively low cutoff frequency will preserve broadcast information while eliminating most of the idle signal, quantization noise, and beats. When such a filter is constructed of analog components, it is difficult and costly to realize. Such a filter must be made of discrete components that tend to be relatively large. Digital filters, on the other hand, can be realized readily in CMOS or other integrated-circuit technologies, but a digital filter would not be satisfactory for placement in a location where the analog signal or an approximation to it has been recovered.
It is an object of the present invention to provide digital filtering in the recovery of a CVSD signal.
It is a further object of the present invention to provide a CVSD receiver with pulse filtering that can be realized on a chip.
Other objects will become apparent in the course of a detailed description of the invention.